In an injection molding system, melt is delivered under pressure from a machine nozzle to a melt channel of a manifold through a sprue bushing. The melt is typically forced through the machine nozzle by an extruding screw. The melt is injected from the manifold into a multi-cavity mold via a plurality of hot runner nozzles. The nozzles include nozzles channels, which are in communication with the melt channel of the manifold.
When the melt flow from the machine nozzle is shut off following the injection of a shot, residual pressures remain in the melt channel of the manifold and the nozzle channels of the nozzles. This residual pressure is undesirable because it may prevent a clean tear between the molded part and the melt in the nozzle. Further, the residual pressure may result in melt drooling from the nozzles when the mold is opened. Drooling is a widespread problem in injection molding and may occur in one face molds, stack molds or tandem molds, for example.
“Suck-back” is a common technique for reducing the pressure in a melt stream following a shot. Suck-back involves creating of a space within the melt channel to artificially reverse the flow of melt for a short period of time. Suck-back is typically achieved by reversing the turning direction of the extruding screw, as described in U.S. Pat. No. 3,902,665 to Hendry, which is incorporated herein by reference.
In injection molding systems in which the machine nozzle must be retracted from the mold in order for the mold to be opened, such as stack mold systems, for example, decompression of the melt using suck-back from the machine nozzle is either not possible or it causes an increase in the cycle time because the suck-back step must be performed prior to the retraction of the machine nozzle.
An alternative attempt to combat the effects of residual pressure in the melt stream following a shot includes adding a decompression mechanism that operates independently from the machine nozzle. A decompression device that extends through a portion of the melt channel of the manifold is disclosed in U.S. Pat. No. 6,348,171 to Dewar et al. Such anti-drool mechanisms introduce flow restrictions into the system, which increase the difficulty of decompressing the melt stream. Further, adding such an anti-drool device to an existing system is prohibitively expensive, as the injection molding apparatus would need to be invasively retrofitted.
It is therefore an object of the present invention to provide a decompression device for an injection molding apparatus that may be added as a modular component to existing multi-cavity hot runner and stack mold systems.